1. Field of the Invention
The present invention relates to an apparatus and method for drying a sheet of material, such as wood pulp or paper, where the sheet is passed in heat exchange contact around a plurality of drying drums.
2. Description of the Prior Art
In the processing of wood pulp, after the pulp is chemically and mechanically treated by digesting, washing, screening and bleaching, the pulp slurry is usually made into the form of a continuous sheet, and subjected to one or more preliminary dewatering steps (e.g. by dewatering with gravity and directing the wood pulp through squeeze rollers, etc.). However, since the sheet of pulp or paper even after such dewatering quite commonly has a relatively high water content (in the order of 55% water by total weight), it is quite often desirable to subject the wood pulp or paper to further drying to bring the moisture content of the sheet to a lower level (e.g. 10% moisture by total weight).
A common method of accomplishing this final drying step is to run lengths of continuous pulp or paper sheet over a series of steam heated drying drums. The temperature of the pressurized steam in the drums can be as high as approximately 350.degree. F., and the heat passes from this steam through the cylindrical shell of each drum to be transferred to the pulp or paper sheet, to bring the temperature of the sheet to a sufficiently high level to cause rapid evaporation of the moisture in the sheet.
Pulp and paper machines of this general type have existed in the prior art for many years, and these have usually been rather massive installations, comprising a plurality of large cast iron drums. Even a medium sized installation, designed to dry 500 tons per day of wood pulp, has as many as approximately 50 drums, and each drum in itself is approximately 250 inches long and sixty inches in diameter, with the cylindrical shell of the drum generally being about one inch thick.
With the heat energy to cause evaporation of the moisture in the sheet being supplied by the steam which is injected into the drum, the effectiveness of the drying apparatus is in turn dependent upon the ability of the heat from the steam within the drums to be passed through the cylindrical wall of each drum to the sheet.
Since cast iron is a relatively good conductor of heat (capable of transmitting 27 BTU'S per square foot per hour per degree Fahreheit temperature differential per foot of thickness) and since water is a relatively poor conductor of heat (i.e. 0.42 BTU'S/ sq. ft., hr., .degree.F./ ft., so as to have about one sixty-fifths of the heat conducting capacity of cast iron) it has long been recognized that one of the major obstacles to optimum heat transfer through the cylindrical shell of the drum is the water which condenses from the steam in the drum.
Accordingly, considerable effort has been directed toward improved means for removing the water condensate from the interior of the drying drum. Such condensate removal means are generally referred to as "syphons", and these are generally classified into stationary syphons and rotary syphons. As its name implies, the stationary syphon keeps the same orientation within the drum as the drum rotates around the syphon. The intake end or mouth of the syphon is placed quite close to the interior surface of the drum and moderately off the low center point of the interior of the drum in the direction of rotation of the drum, since the water condensate which tends to form as a "puddle" in the lower part of the drum is shifted from the low point in the direction of rotation of the drum. One of the problems with the stationary syphon is that of placing its intake opening sufficiently close to the interior surface of the drum for optimum water removal.
The rotary syphon alleviates to some degree the problem of placing the inlet of the syphon quite close to the interior surface of the drum, since it remains in the same location relative to the drum. However, since with the rotary syphon the inlet of the syphon necessarily rotates with the drum, it passes through the "puddle" of the water condensate only once for every revolution of the drum. Then even with the rotary syphon, there are practical limitations as to how close the inlet can be placed to the interior surface, since the inlet is subject to clogging or other malfunction if the spacing of the inlet opening to the interior wall surface is too close.
There are varying views as to the relative effectiveness of the rotary and stationary syphons. One study indicates the superiority of a rotary syphon over several types of stationary syphons investigated. Specifically, certain test results are cited in a paper entitled "A Comparison of Rotary and Stationary Syphon Performance in Paper Dryers", by D. L. Calkins, the Johnson Corporation, Three Rivers, Michigan, published in 1973. In that paper, the performance of one rotary syphon was compared with that of three stationary syphons having different inlet configurations. The tests were conducted using a conventional cast iron drying drum, having a diameter of 60 inches and an axial length of 250 inches. By measuring the residual condensate in the drum after operation, it was concluded that the rotary syphon design studied was superior to the several stationary syphons used under the same operating conditions in terms of maintaining the total quantity of water condensate in the drum to a practical minimum. Also, the surface temperature of the drum when the rotary syphon was used was somewhat higher than when the three stationary syphons were used. From those tests, it could reasonably be concluded that with the syphon which more effectively removes condensate from the drum interior in terms of total volume of condensate in the drum at any one time, better heat transfer through the cylindrical wall of the drum can be obtained.
Various devices for the removal of condensate from a drum dryer are shown in a number of U.S. patents. For example, U.S. Pat. No. 1,640,019 discloses a plurality of elongate trough members secured to the inner surface of the drum. Each trough is made of sheet metal which is bent into a helical configuration so that the trough opening is directed in the direction of rotation of the drum. U.S. Pat. No. 977,376, Dodge, and U.S. Pat. No. 1,483,343, Gladin, show other arrangements of this same general concept.
U.S. Pat. No. 2,791,038, Armstrong, shows a scoop member to collect water which then passes through a slot. There is a groove-like sump formed axially in the drum inner surface to collect water.
U.S. Pat. No. 2,420,824, Hornbostel et al., shows a collecting trough which is spaced slightly from the interior surface of the drum. The alleged purpose of this configuration is to permit uniform heating along the surface of the drum.
U.S. Pat. No. 3,513,565, Jacobson, shows a device to remove the moisture from the drum where there are a plurality of small tubes which extend radially from a manifold. These tubes are spaced along substantially the entire axial length of the drum, and there are suction tubes to draw the collected water condensate into the center shaft or axle of the drum. The intention of this arrangement is also to provide a more uniform temperature at the drying surface of the drum.
A different approach to the problem of obtaining improved heat transfer through the wall of the drum is disclosed in U.S. Pat. No. 3,426,839, Overton. The steam which is injected into the drum is discharged through jets closely adjacent the inner surface of the cylinder wall to mechanically scour by means of the jets of steam the inner surface of the cylinder wall to break up and remove condensate film. However, the patent states that this requires the use of more steam than can be completely condensed in the cylinder for purposes of providing adequate heat for the drying of the material on the exterior of the drum. Further, since the steam is discharged as jets under pressure, it can be presumed that it is necessary to deliver the steam to the jet nozzles at a pressure higher than that which exists in the interior of the drum.
While many of the improvements in the prior art have improved the heat transfer capability or other characteristics of the prior art drums, there is a continuing need for improvement to enhance the operating effectiveness of such drying equipment. Thus, it is an object of the present invention to provide a method and apparatus for drying material such as a sheet of wood pulp or paper, and specifically to enhance the effectiveness of heat transfer through the walls of a steam heated drum drying apparatus.